This PR pulls the upstream change, Use accessors for ValueHandleBase::V; NFC (llvm/llvm-project@6f08789), into DXC.

Here's the summary of the change:
  This changes code that touches ValueHandleBase::V to go through getValPtr and (newly added) setValPtr.  This functionality will be used later, but also seemed like a generally good cleanup.

  I also renamed the field to Val, but that's just to make it obvious that I fixed all the uses.

This is part 1 of the fix for #6659.

This PR pulls the upstream change, Emulate TrackingVH using WeakVH (llvm/llvm-project@8a62382), into DXC.

Here's the summary of the change:

  This frees up one slot in the HandleBaseKind enum, which I will use later to add a new kind of value handle.  The size of the HandleBaseKind enum is important because we store a HandleBaseKind in
  the low two bits of a (in the worst case) 4 byte aligned pointer.

  Reviewers: davide, chandlerc

  Subscribers: mcrosier, llvm-commits

  Differential Revision: https://reviews.llvm.org/D32634

This is part 2 of the fix for #6659.

This PR pulls the upstream change, Rename WeakVH to WeakTrackingVH; NFC (llvm/llvm-project@e6bca0e), into DXC.

Here's the summary of the change:

  I plan to use WeakVH to mean "nulls itself out on deletion, but does not track RAUW" in a subsequent commit.

  Reviewers: dblaikie, davide

  Reviewed By: davide

  Subscribers: arsenm, mehdi_amini, mcrosier, mzolotukhin, jfb, llvm-commits, nhaehnle

  Differential Revision: https://reviews.llvm.org/D32266

This is part 3 of the fix for #6659.

This PR pulls the upstream change, UAdd a new WeakVH value handle; NFC (llvm/llvm-project@f1c0eaf), into DXC.

Here's the summary of the change:

  WeakVH nulls itself out if the value it was tracking gets deleted, but it does not track RAUW.

  Reviewers: dblaikie, davide

  Subscribers: mcrosier, llvm-commits

  Differential Revision: https://reviews.llvm.org/D32267

This is part 4 of the fix for #6659.

This PR pulls the upstream change, Use a 2 bit pointer in ValueHandleBase::PrevPair; NFC (llvm/llvm-project@b297bff), into DXC.

Here's the summary of the change:

  This was an omission in r301813.  I had made the supporting changes to make this happen, but I forgot to actually update the PrevPair declaration.

  llvm-svn: 301817

This is part 5 of the fix for #6659.